Adenosine deaminase (ADA) is an enzyme of purine metabolism that displays a fascinating pattern of tissue specific distribution and developmental regulation. The highest levels of ADA activity and the most pronounced developmental control are found in the stomach and the thymus where enzyme levels are approximately one hundred times that of most other tissues. Although genetic, biochemical and pharmacologic evidence clearly indicate that ADA plays an essential role in the development and function of the immune system, a physiological role for the enzyme in the stomach has not yet been identified. My colleagues and I have chosen the mouse as a model system to address a number of questions regarding ADA gene structure, expression and developmental regulation. In order to obtain the molecular reagents necessary for our studies, we have isolated mouse cell lines in which ADA levels are increased over 11,000fold and in which enzyme accounts for over 75% of the soluble protein. The phenotype of these cells results from the amplification of functional ADA genes which account for approximately 10% of the genome. The availability of these cells has enabled us to purify large amounts of the enzyme to homogeneity, to prepare monospecific antisera, to obtain fulllength and functional copies of ADA cDNA, and to isolate genomic clones encompassing the entire ADA structure gene. Proposed experiments will involve the utilization of these molecular reagents to address the following questions concerning the developmental regulation of ADA gene expression in mice. What is the tissue and cellular distribution of ADA activity in mice and how does this change as a function of development. Are differences in ADA mRNA abundance and/or structure associated with tissue or cell specific differences in the level of ADA enzyme? Does the level of ADA transcriptional activity account for tissue and cell specific differences in ADA mRNA abundance? What are the cis- regulatory elements associated with proper cell specific and developmental regulation of ADA gene expression?